Birdman- here's the basic quick and dirty summary of method to the madness for dump zones for wood boilers--
a 20th century fossil-fueled boiler, as most homeowners/ designers/ plumbers are used to, controls the overall heat output by turning the flow of fuel (gas or oil) on and off; the air (oxygen) goes along for the ride with the flow of the fossil fuel, in order to make the combustion happen. Oxygen/ air is basically abundant (thankfully, so we all can breathe
, thank the plants
) and so the whole thing is controlled by turning the fuel on and off, which can be done easily and rapidly.
a wood boiler, of any type, whether high efficiency (gasifier) or low efficiency (OWB or older wood boiler) represents a box full of fuel, surrounded by a water jacket, and then you try (with, sometimes, no small amount of effort and consternation) to control the oxygen for the combustion to regulate how hard/fast that box of wood fuel burns and belches heat into your desired areas to be heated. And all that air and oxygen, which we're all glad is abundant so that we can breathe and live, is hard to keep out of that big box of already-burning wood fuel.
then, with either heat source, you have your whole 'nuther system built to take heat from the fire to the living area; in 20th and 21st century heat systems, those depend on electrically-powered pumps and valves to move the heat, and those things shut off instantly when the power goes down.
with the 20th century boilers (gas or oil), when the power fails, the pumps and/ or valves feeding the fuel shut down, right away, and so the fire in the box goes out- FAST - there's no extra heat, or for very long. the pumps and motors that normally move the heat to the living space shut down, too, but it's no big deal, because the fuel source has simultaneously shut off
with the solid-fueled gasifiers, when the power goes out-- during a burn--, you've still got a box full of fuel, and, even if the electric blower supplying abundant lotso' air to the fuel shuts down from the power outage, you've still got a box of burning fuel that is in the middle of burning, and won't change its mind quickly-- and will pull air in via the air inlet and the draft of the chimney, even without the powered blower, so you need to give all that (substantial and longer-lasting) heat a safe place to go, so that the wood boiler can get a "soft landing" so that no spots within the boiler hit full boil, flash from water to steam, and create pressures that would rapidly wreck something.
with that in mind, there is, within relative reason, no such thing as "too big" a gravity flow dump zone.
you do not need to design for total overkill, but, subject to that basic sensibility, you do not need to worry about your fail-safe dump zone being "too big"
hope that helps